Lithium secondary batteries, which have high electromotive force and high energy density, have been used as the main power source for electronics such as mobile communications devices and portable devices and as the power source for memory back-up. The demand for lithium secondary batteries is increasing year after year. As electronic devices are becoming smaller, more sophisticated, and maintenance-free, non-aqueous electrolyte secondary batteries are required to provide higher capacities and improved storage characteristics. Also, clock back-up power sources for digital still cameras, etc. are particularly required to have improved storage characteristics under high temperature environments since they may be exposed to high temperatures during the operation of devices.
Lithium-containing manganese oxides are widely used as positive electrode materials, but they have a problem in that dissolution of manganese therefrom during storage at high temperatures causes the battery performance to degrade. To suppress the dissolution of Mn during storage at high temperatures, for example, Japanese Laid-Open Patent Publication No. Hei 11-071115 proposes replacing a part of Mn contained in a lithium-containing manganese oxide with another transition metal element M such as Ni or Fe. Also, for example, Japanese Laid-Open Patent Publication No. Hei 10-172571 proposes positive electrode active material particles comprising a lithium-containing manganese oxide in which a part of Mn is replaced with another transition metal element M such as Ni or Fe. The proposed active material particles have a two-layer structure composed of a central layer and a surface layer, and the central layer and the surface layer have different Mn valences.
However, when a positive electrode material described in Japanese Laid-Open Patent Publication No. Hei 11-071115 or No. Hei 10-172571 is used, the capacity may decrease. That is, when a part of Mn contained in a lithium-containing manganese oxide is replaced with a transition metal element M, the practical battery capacity may decrease due to a change in the relation between the positive electrode potential and the capacity. This is probably because the potential at which the valence of the transition metal element M changes is different from that for manganese. Also, when the amount of the transition metal element M is reduced to avoid the capacity decrease, the dissolution of Mn cannot be sufficiently suppressed.
In order to solve the above-discussed problems associated with conventional techniques, it is therefore an object of the invention to provide a Mn-containing positive electrode material for a non-aqueous electrolyte secondary battery in which the dissolution of Mn during storage at high temperatures is suppressed without lowering the positive electrode capacity. Another object of the invention is to provide a high capacity battery with excellent high temperature storage characteristics by using the positive electrode material. Still another object of the invention is to provide an easy method for producing the positive electrode material for a non-aqueous electrolyte secondary battery.